1. Field of the Invention
The present invention relates to a photoelectric converter and a radiation reader, more particularly to a photoelectric converter capable of performing the read operation at higher sensitivity and higher speed and a radiation reader capable of reading the information for radiation represented by (xcex1-rays, xcex2-rays, xcex3-rays or x-rays at a high sensitivity.
2. Related Background Art
In the case of a photoelectric converter and a radiation reader for reading the information based on radiation by wavelength-converting the radiation into the sensitivity region of the photoelectric converter with a wavelength converter such as a photoluminescent body (for example, scintillator), electric charges based on the input information photoelectric-converted by a photoelectric-converting section is transferred to a capacitance to amplify a signal voltage.
However, to amplify a signal voltage by transferring electric charges from the capacitance of a photoelectric-conversion element itself to an external capacitance like the case of reading signal charges with a conventional circuit, a relatively-large S/N ratio can be obtained but a parasitic capacitance may be formed on a signal line when arranging a plurality of sensors. For example, when arranging 2,000xc3x972,000 area sensors equivalent to an X-ray film at a size of 200xc3x97200 xcexcm per cell and manufacturing an area sensor with a size of 40xc3x9740 cm, a capacitance is formed because the gate and source of a transistor for transferring electric charges are overlapped each other. Because the above overlap corresponds to the number of pixels, one signal line has an overlap Cgs capacitance of 0.05 pFxc3x972,000 area sensors=100 pF even if one area sensor has a capacitance of approx. 0.05 pF. Because a sensor capacitance Cs is equal to approx. 1 pF, when assuming a signal voltage generated in a sensor as V1, the output voltage V0 of the signal line is obtained from the following expression.
V0=(Cs/(Cs+Cgsxc3x971000))xc3x97V1
Thus, the output voltage lowers to 1/100.
That is, to constitute an area sensor having a large area, an output voltage is greatly lowered.
Moreover, to read a dynamic image under the above condition, a sensitivity and a high-speed operation capable of reading 30 images per sec. or more are requested. Particularly, in the case of a nondestructive inspection including X-ray diagnosis, higher sensitivity allowing the number of signal charges to increase up to 100 to 400 times is requested because there is a request for minimizing the dose of X-rays to be irradiated.
The present invention is made to solve the above problems and its object is to provide a photoelectric converter capable of preventing an output voltage from lowering due to increase of a parasitic capacitance and resultantly having higher sensitivity and more-advanced performance and a radiation reader having the photoelectric converter.
It is another object of the present invention to provide a photoelectric converter having a large opening ratio, that is, a large rate of the light-receiving-section region in an area necessary for one pixel and resultantly capable of achieving a high sensitivity and an advanced performance and a radiation reader having the photoelectric converter.
It is still another object of the present invention to provide a photoelectric converter capable of reading a dynamic image.
It is still another object of the present invention to provide a radiation reader capable of further reducing the dose of radiation such as X-rays.
It is still another object of the present invention to provide a photoelectric converter having a semiconductor layer comprising in one pixel: a photoelectric conversion element, a reading field-effect transistor having a gate for receiving signal charges generated in the photoelectric conversion element and a source and a drain for reading a signal corresponding to the signal charges accumulated in the gate, selection-switch means set between the reading field-effect transistor and a power supply, and reset means for resetting the gate; wherein
the photoelectric conversion element, the reading field-effect transistor, the selection-switch means, and the reset means are formed on a common insulating support body.
It is still another object of the present invention to provide a radiation reader comprising a photoelectric converter having a semiconductor layer comprising in one pixel: a photoelectric conversion element, a reading field-effect transistor having a gate for receiving signal charges generated in the photoelectric conversion element and a source and a drain for reading a signal corresponding to the signal charges accumulated in the gate, selection-switch means set between the reading field-effect transistor and a power supply, and reset means for resetting the gate; wherein the photoelectric conversion element, the reading field-effect transistor, the selection-switch means, and the reset means are formed on a common insulating support body; and a photoluminescent body set on the photoelectric conversion element to absorb radiation and emit light in a wavelength band which can be detected by the photoelectric conversion element.
It is still another object of the present invention to provide a radiation reader having the following in one pixel: a radiation reading element which includes an electric-charge discharging layer for absorbing radiation and discharging electric charges and in which at least the electric-charge discharging layer is constituted so as to be held by two conductive layers, a reading field-effect transistor having a gate for receiving signal charges generated in the radiation reading element and a source and a drain for reading a signal corresponding to the signal charges accumulated in the gate, selection-switch means set between the reading field-effect transistor and a power supply, and reset means for resetting the gate.